Type the server's NoLeaderException instead of a misspelled key

[poissoncorp]

The server sends NoLeaderException. The map was keyed on NoLoaderException, a name the server never sends, so a real no-leader failure reached callers as a plain RavenException.

schema.type = "Raven.Client.Exceptions.Cluster.NoLeaderException"
ExceptionDispatcher.get(schema, 500)
# before: RavenException
# after:  NoLeaderException

The class is renamed to match, with the old name kept as an alias so existing imports and except clauses keep working:

NoLoaderException = NoLeaderException

Three tests in ravendb/tests/issue_tests/test_exception_dispatcher.py, no server needed.

Found while syncing the test driver with C#, where teardown has to swallow a no-leader failure and could not catch it by type.